Nice find; it's Acteon tornatillis, described as of local distribution, & even, in Hayward's seashore, as a "rather rare creature sporadically distributed on sandy coasts of western Britain". It feeds on polychaete worms like Lanice conchilega. Rhossili's known to be a good place for it, often as empty shell or occupied by a hermit crab, but at low tide you can also find the live animal there on the sandy flats, & other places including Swansea bay.